A grain oriented electrical steel sheet is mainly utilized as an iron core of a transformer and required to exhibit superior magnetization characteristics, e.g. low iron loss in particular.
In this regard, it is important to highly accumulate secondary recrystallized grains of a steel sheet in (110)[001] orientation, i.e. what is called “Goss orientation”, and reduce impurities in a product steel sheet. However, there are restrictions on controlling crystal grains and reducing impurities in view of production cost. Accordingly, there has been developed a technique of introducing non-uniformity into a surface of a steel sheet by physical means to subdivide width of a magnetic domain to reduce iron loss, i.e. magnetic domain refinement technique.
For example, JP-B 57-002252 proposes a technique of irradiating a steel sheet as a finished product with laser to introduce linear, high-dislocation density regions into a surface layer of the steel sheet, thereby narrowing magnetic domain widths and reducing iron loss of the steel sheet. The magnetic domain refinement technique using laser irradiation of JP-B 57-002252 was improved thereafter (see JP-A 2006-117964, JP-A 10-204533, JP-A 11-279645 and the like), so that a grain oriented electrical steel sheet having good iron loss properties can be obtained.
Further, JP-A 2000-119824 discloses an experiment example of improving iron loss by laser irradiation as a method for improving iron loss properties of a steel sheet in a component system not using an inhibitor (i.e. an inhibitorless component system). Yet further, JP-A 2007-138201 discloses an example of reducing iron loss of a steel sheet by specifying a titanium compound added to an annealing separator and annealing atmosphere during final annealing when an inhibitorless steel material is used.
Various technical improvements have been made for the magnetic domain refinement technique as described above. However, there is a demand for further improvement of iron loss properties of a grain oriented electrical steel sheet due to increasing public awareness of energy-saving and environment protection in recent years.
However, none of the grain oriented electrical steel sheets disclosed in JP-B 57-002252, JP-A 2006-117964, JP-A 10-204533, JP-A 11-279645 can achieve sufficiently low iron loss values satisfying such a public demand as described above. Further, JP-A 2000-119824 and JP-A 2007-138201 also have such problems as described below.
That is, JP-A 2000-119824 improves iron loss properties by restricting Al content in steel, but pays no attention as to how compounds in forsterite coating (coating mainly composed of Mg2SiO4) affects laser irradiation and fails to obtain a sufficient magnetic domain refinement effect by laser. A sufficient magnetic domain refinement effect by laser cannot be obtained solely by the controlling techniques described in JP-A 2007-138201 as well.